How Special Relativity Makes Magnets Work

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    Magnetism seems like a pretty magical phenomenon. Rocks that attract or repel each other at a distance - that's really cool - and electric current in a wire interacts in the same way. What's even more amazing is how it works. We normally think of special relativity as having little bearing on our lives because everything happens at such low speeds that relativistic effects are negligible. But when you consider the large number of charges in a wire and the strength of the electric interaction, you can see that electromagnets function thanks to the special relativistic effect of length contraction. In a frame of reference moving with the charges, there is an electric field that creates a force on the charges. But in the lab frame, there is no electric field so it must be a magnetic field creating the force. Hence we see that a magnetic field is what an electric field becomes when an electrically charged object starts moving.

    I was inspired to make this video by Prof. Eric Mazur

    Huge thank you to Ralph at the School of Physics, University of Sydney for helping us out with all this magnetic gear. Thanks also to geology for loaning the rocks.

    This video was filmed in the studio at the University of New South Wales - thanks to all the staff there for their time and support.

    Music: Firefly in a Fairytale, Nathaniel Schroeder, and Love Lost (Instrumental) by Temper Trap licensed from
  • Video CategoriesEducation
  • Runtime: 4:19
  • Tags for this video:  veritasium  science  physics  magnet  magnets  magnetic  how do electromagnets work?  how do magnets work?  special relativity  length contraction  attract  repel  minutephysics  charged cat  

Comments: 4 210

  • Bartosz Olszewski
    Bartosz Olszewski 47 minutes ago

    his logo stands for V(ap)e 4:20 lol
    im so dumb

  • Andrew Combs
    Andrew Combs 3 hours ago

    My head hurts in the most wonderful way possible...

  • StarlightVisual
    StarlightVisual 5 hours ago


  • George K
    George K 1 day ago

    no force on a kitty. <-- this should be the leading comment

  • Marco Discendenti

    Are really wires electrically neutral in general?

  • bhargaw gaikwad
    bhargaw gaikwad 2 days ago

    if special relativity is the reason then charge must attract in your frame of reference as the moving electrons suffer from length contraction.My friend I read many books but no one give explanation of electromagnetism through special theory of relativity.
    please check the hypothesis.

  • Rajat soni
    Rajat soni 4 days ago

    love it

  • Sander Wildeman
    Sander Wildeman 5 days ago

    The video is also quite confusing to me. Maybe the dilution of the electrons (from the point of view of the cat) can also be viewed as an effect of time dilation.

    If we imagine the full circuit, i.e. including the battery which pushes out the electrons (for simplicity assume that the electrons are injected from the left and absorbed on the right), then in the frame of the cat the "battery" is moving and hence its "clock" runs slow. In other words, as far as the cat is concerned the battery pushes out electrons at a lower rate, and hence the density is less.

    The distance between ions is contracted (along with the battery and the rest of the circuit, and including the space between the electrons) as explained in the video.

    Also note that in the example the magnetic field according to the cat is not zero, as there is still a current of ions. So it is not true that the magnetic field simply gets "converted" into an electric field and so it is not true that a magnetic field is "explained" by special relativity. It just means that "the" electric and magnetic field are relative concepts (depending on your motion), just like space and time.

    Just my ten cents to add to the confusion :)

  • Josh Richter
    Josh Richter 5 days ago

    why is it that electromagnets can act upon stationary objects as well? (not just moving ones)

  • Hyperflip
    Hyperflip 5 days ago

    Woah, after learning about both elecromagnetism and special/general relativity for my final exam, this is such a mindblowing connection!

  • Saw Teck Rul
    Saw Teck Rul 6 days ago

    thanks Henry and Derek. :D

  • David Zare
    David Zare 6 days ago

    sorry the explanation is sooooo wrong from beginning to the end. not to mention that regardless of protons/electrons charge neutrality every moving charge is a source of magnetic field.

  • Paul Anderson
    Paul Anderson 6 days ago

    I still can't wrap my head around this. - Why do things change? I can't get past the fact that it seems that we're only talking about perception. Not reality. Just perception.

  • Paul Anderson
    Paul Anderson 6 days ago

    watched this twice...head still hurts...

  • Professional Insultant

    are you trolling us?

  • charles joshi
    charles joshi 8 days ago

    awesome video sir, thank u so much

  • Siddarth Arcot
    Siddarth Arcot 8 days ago

    The "Only in your frame of reference" made me laugh idk why

  • Sean Munro
    Sean Munro 8 days ago

    So, in the real world, what are the cats? E.g. Are the protons in the compass the cats? How and why do they move when he sends current through the wire? :\

  • Nikhil Joshi
    Nikhil Joshi 8 days ago

    Question and a possible experiment : If the magnet and or the cat is itself shaped like a wire and of the same length as wire, will it still experience the repulsive force ?

  • Math
    Math 10 days ago

    @ 3:23- Therefore, electrons drift through wires at about 5.9 or 5.86 miles per month

  • Noel Ruppenthal
    Noel Ruppenthal 10 days ago

    The compass needle is neither moving with the direction of current in the wire, nor is positively charged, so I'm not seeing how length contraction is responsible for the wire deflecting the compass needle

  • Akshay Kumar
    Akshay Kumar 11 days ago

    In our reference frame, charged cat is stationary and some current is in wire, current means electron is moving and positive charge is stationary. If electron is moving, distance between electron will contract. and it will produce some electric field that will attract the positive charged cat. Why this is not happening?

  • Istasis Mishra
    Istasis Mishra 12 days ago

    In Derek's frame of reference, since the electrons are moving, the electrons should seem like they have contracted and the wire should seem positively charged and this accounts for the repulsion of cat. Why isn't this the case?

  • AstralPotatoe
    AstralPotatoe 13 days ago

    my mind is blowned, magnetic fields are a lie, they are just electric fields

  • Saad Selwan
    Saad Selwan 14 days ago

    why is a stationary magnet attracted to a copper wire with a current flowing in it ?

  • Orion Michael Guy
    Orion Michael Guy 16 days ago

    Its the Motion of the Earth that gives the Magnet its energy

  • Aaron Forbes
    Aaron Forbes 16 days ago

    there were way too many different metaphors/similes in this to make coherent sense for me

  • Simon Barry
    Simon Barry 17 days ago

    well said

  • Chandan Bhuyan
    Chandan Bhuyan 18 days ago

    I am bit confused can anyone help.?
    1)when cat is stationary & electrons were moving then from cats reference due to length contraction electron density increases and proton spaced out making overall - ve charge which should attract the cat.. But didn't
    2) In this video a positive object which is
    Moving at speed of electron is repelled by electromagnet but what about stationary object or - ve charge object

  • Nosknut
    Nosknut 18 days ago

    Almost quote 1: "No force on the stationary cat next to a wire conducting electricity."

    Almost quote 2: "Force on a stationary cat next to a wire conducting electricity."


  • Science Asylum Fan
    Science Asylum Fan 18 days ago

    Why moving magnets move the electrons in a conductor? A moving magnet induces current in a conductor.Why?

    Changing magnetic field creates a current in a conductor.What is the reason at the microscopic scale. Explain without any technical jargon?

  • יאיר ערד
    יאיר ערד 19 days ago

    now I will pretend that I understood this video

  • Atif Wahab
    Atif Wahab 21 day ago

    Why don't electrons contract in the first case? Because their motion in reality is random?

  • Jon Do
    Jon Do 21 day ago

    well i think quantum Physicians would expect the universe to split. for the cat the wire is positive so in its universe it gets pushed away. And you should enter a universe there the cat does not get pushed away.

  • sanmveg saini
    sanmveg saini 23 days ago

    To all those who are asking that why that cat wasn't attracted when it was at rest (or more specifically, moving with the velocity of the positive ion in the wire, if any ), then just remember that in a neutral wire (a wire with no current in it) has electrons moving with random velocity, yeh! the velocity (speed+direction). and the number density is so high that you will always find two electrons moving with velocity in opposite direction in a particular volume of wire, and as you have heard that current is the flow of electrons(very basic definition ), so over all there is no current, hence neutral implies no force.

    although i do not know special theory of relativity in detail, but from the given explanation we figure out that, in the video he said that the length got contracted IN DIRECTION OF MOTION (not shouting just highlighting), so two electron will move in opposite direction and hence length will contract in opposite direction hence no net change in the length hence there is no change in the number density of the electrons hence no net force.

    hope you understand, have a grate day!

  • abc def
    abc def 25 days ago

    you need to come back with the correct answer, at 2:26 . moving electrons didnt get the length contraction, this is an inconsistancy, illiogical.

  • Landon Fitzgerald
    Landon Fitzgerald 25 days ago

    DEREK CAN YOU MAKE ANOTHER VIDEO CLARIFYING THIS PLEASE?? We're confused here because the car analogy doesn't pair perfectly with the stationary electromagnet. What is the car representing? You had no moving objects other than a current that generated an attractive force in your electromagnet. Additionally, why don't the electrons contract? Can you explain this using BOTH electrical motors and generators? Any help is much appreciated, thanks!

  • Landon Fitzgerald
    Landon Fitzgerald 25 days ago

    Please help!! It's been three days and I still can't find any good material on this. Is the cat a magnet? Why are things attracted to the electromagnet when the current is turned on?

  • Science Asylum Fan
    Science Asylum Fan 27 days ago

    😍 love you so much Ve- The element of truth

  • Aabhas Dhaubanja
    Aabhas Dhaubanja 27 days ago

    so does this mean that magnets are affected by charges, because since the density of proton increases and the wire becomes positive the magnet in the compass is deflected..plz help

  • SoCloseToTheSky
    SoCloseToTheSky 27 days ago

    ehm, I thought magnets are not that freaking complicated.

  • Martin Ducharme
    Martin Ducharme 29 days ago

    I really like cooperative videos between youtubers.

  • Benjamin Lehmann
    Benjamin Lehmann 29 days ago

    Best, most mind blowing video I've seen on YubTubs for a year.

  • José B. Junior
    José B. Junior 1 month ago

    Einstein still blowing my mind.

  • Caue Rego
    Caue Rego 1 month ago

    what would happen to physics if instead of using cats to explain theories they'd use armadillos?

  • kristoffer gaard
    kristoffer gaard 1 month ago

    I hope somebody can answer this question:

    According to special relativity, objects moving relative to one-another are subject to length contraction. So when the cat is static and the electrons in the wire start moving, wouldn't the electrons be subject to length contraction? Thus creating a higher density of electrons thus attracting the kitty?

  • ski
    ski 1 month ago

    I really think a video about drift velocity would clarify this topic even more so than this excellent clip already does.

  • kdmc40
    kdmc40 1 month ago

    Ah! This explains why I'm attracted to my sister-in-law! I knew it wasn't my fault!

  • Sean Dafny
    Sean Dafny 1 month ago


  • Sean Dafny
    Sean Dafny 1 month ago

    Thank u so much Feynman

  • SillieWous
    SillieWous 1 month ago

    In the stationary cat case: why is the cat not attracted to the wire? Moving electrons would be more densely packed, thus overall negative charge and hence the cat should be attracted.

  • O Gor
    O Gor 1 month ago

    it's a shame. delete this video

  • ​ ​
    ​ ​ 1 month ago

    For anyone that came there trought the question "Why is magnetic force perpendicular to current/charge velocity/magnetic field?", this video perfectly explains.

  • chari Muvilla
    chari Muvilla 1 month ago

    So when the electrons are moving it is like there are more of them everywhere?! Lol I'm never gonna understand how magnets work. Stupid world

  • Nikhil Aggarwal
    Nikhil Aggarwal 1 month ago

    why not non-moving cat is attracted to wire?

  • Diluksh Fernando
    Diluksh Fernando 1 month ago

    0:45 Adolf Hitler

  • Rober oat
    Rober oat 1 month ago

    So how does this attract ferromagnetic stuff

  • Name_less
    Name_less 1 month ago

    So if I'm moving fast enough i could occupy less space that someone who is stationary, what if blackholes are just moving really fast therefore taking up much more space than they seem to be, so it can look like they are squeezing matter into a fixed position to make a really strong gravitational force, but they are really just spinning really fast.

  • Minecraftster148790
    Minecraftster148790 1 month ago

    Does this mean you can determine the speed of light using an electromagnet?

  • Ido Ben Lulu
    Ido Ben Lulu 1 month ago

    why the density of the electron proton be the same if the electons move ? Dont the electons suppose to cover less space and hence the wire would be negatively charge from Vertasium reference? why only when the protons "move" the density isnt the same?

  • Samantha Kun
    Samantha Kun 1 month ago

    kawaii cat <3

  • Jack 'o Bong
    Jack 'o Bong 1 month ago

    this video shows how magnets are RELATED to RELATIVITY

  • Johnny Pacheco
    Johnny Pacheco 1 month ago

    I think you mix thinks together which have nothing to do with one another.....

  • Sam Sırri
    Sam Sırri 1 month ago

    02:13 this cat has a badly inflamed tooth root :(

  • Victor Drevin
    Victor Drevin 1 month ago

    So an electron only moves about three centimetres per second?

  • Ajas Azeez
    Ajas Azeez 1 month ago

    how the attraction between an iron piece and an electro magnet relatively at rest can be explained by the same theory

    • Sam Sırri
      Sam Sırri 1 month ago

      the electrons are still moving in the wire.

  • onehit pick
    onehit pick 1 month ago

    A positive cat experiences forces with a neutral wire with no current, including gravitational and electromagnetic forces. Your opening statements contradict the simplest of experimental observations and the most fundamental of theories. Try rubbing a balloon on your hair and see how it interacts with nearby mostly neutral objects, including ceilings, wire strands, etc...

  • martiarenax3
    martiarenax3 1 month ago

    Bad. From this point of view the electromagnets should be purely one negative field and it's not so.

  • Joebert
    Joebert 1 month ago


  • Prince 1022
    Prince 1022 1 month ago

    why is special relativity true ?

  • Hebert Sanchez
    Hebert Sanchez 1 month ago

    I am in an infinite magnetic loop between veritasium and minute physics. :(

  • Lars Berntzon
    Lars Berntzon 1 month ago

    Can you explain how the magnetic spin of the electrons comes into play for the relativistic effect? I don't understand this.

  • Ariel Nicolai
    Ariel Nicolai 1 month ago

    Very interesting!. The example is understood. but now, someone can explain me what happen if the cat stays still?
    1. If I think the example using the traditional phisics: the cable makes a magnetostatic field and the cat doen't experiment a force because its velocity is null.
    2. But if I think the example using the relativistic phisics: the electrons are moving into the cable, so the cat sees that the electrons are closer each other because the contraction of lenght. So, the resultant charge is negative (if I close a section the cable in a volume) giving an electric resultant field and so, a resultant electrical force in the cat ponting to the cable...

    Please help me, because I can't stop thinking! jajaja

  • GrigoriYefimovichRasputin

    Holy crap! What a beautiful and simple explanation. The "only in your frame of reference was hilarious" 10/10

  • Ariel Nicolai
    Ariel Nicolai 1 month ago

    Dear veritasium, what is the name of the music in background???

  • Ashwini Kumar
    Ashwini Kumar 1 month ago

    Why is there no force on cat when only current is flowing through the wire?
    electrons are flowing relative to the cat , so it should contract as it is moving.
    And hence the wire should be more negative and cat should get attracted towards the wire.

  • Marcus Tornea
    Marcus Tornea 1 month ago

    For the cat's stationary frame, the electrons should be denser therefore it should attract the cat. But for the electron's frame, the positive charges are denser; therefore it shouldn't attract the stationary cat. For the cat's moving frame, the positive charges look denser; therefore it repels the cat, but for the positive charges' frame, the electrons look denser and therefore should instead attract the moving cat. Knowing that relativity states that all perspectives are considered valid, I personally disagree with this theory.

  • Shubham Dhiman
    Shubham Dhiman 1 month ago

    mind = blown !
    why is this not taught in relativity courses?

    • Shubham Dhiman
      Shubham Dhiman 1 month ago

      0MoTheG of course that is taught but the fact that SR unites electricity and magnetism isn't even mentioned

    • 0MoTheG
      0MoTheG 1 month ago

      The Lorentz transformation is taught in every course that includes SRT.

  • Shree Vignesh
    Shree Vignesh 1 month ago

    So if there is a stationary positive charge outside the wire it gets attracted when current passes through the wire??

  • Rakibul Islam Prince

    yesssssssssssssssss..........i got it........
    thanks bro...................thank u very much

  • Vishal Shah
    Vishal Shah 1 month ago


  • breakingthemasks
    breakingthemasks 1 month ago

    never commented here before...but this video and it's twin were stellar. more high level videos! great job🖒

  • Alexander Dräbenstedt

    So does an electron beam, which is a current without surrounding protons, not produce a magnetic field? I would doubt that.

  • Ryan Koepke
    Ryan Koepke 2 months ago

    what is the positive "cat" was moving near the speed of light? What would that do to the charge?

  • makro80
    makro80 2 months ago

    How does the cat near the neutral wire at 3:00 get pushed away? Like he said in the video its due to the magnetic field around the wire but I dont understand how that would cause it to be repelled?

  • nikitha nayaer
    nikitha nayaer 2 months ago

    mind = blown
    oh and how did you assume the cat was female what if it identified it's self as a helicopter

  • Cat Man
    Cat Man 2 months ago

    The best explanation I've seen for electromagnets

  • Engineer Ahmad
    Engineer Ahmad 2 months ago

    Direction of Magnitic field is tangent to conductor not away from it

  • Kyle MacDonald
    Kyle MacDonald 2 months ago

    How do you grow such a thick beard in six hours? Takes me a week and a sacrifice to the eldritch follicular powers.

  • peanutaxis
    peanutaxis 2 months ago

    The velocity of an electron in a 2mm radius wire with 1 ampere flowing throuogh it is:


    .....and I'm supposed to believe that relativistic effects are relevant. I don't *&%ing think so.

    wiki: Drift_velocity#Numerical_example

  • erikk77
    erikk77 2 months ago

    Thank you Michael Faraday.

  • Dercio Silveira
    Dercio Silveira 2 months ago

    I dreamed that a moving charged particle affect some quantum field like plane´s wings, creating lifting (magnetisim)...

  • Cold Death By Multiverses

    This all sounds more like a classic interpretation rather than a relativistic one, because only Galilean Relativity was used to explain it.

  • deltango12345
    deltango12345 2 months ago


  • Mike Billars
    Mike Billars 2 months ago

    This doesn't make sense to me. I always thought that electrons actually move *very* slowly through a conductor - like millimeters per second. If that's true, then the Lorentz contraction would be so insignificant that I can't imagine it having any effect at all. What am I missing?

  • Lu Chee Ket
    Lu Chee Ket 2 months ago

    What consequent can we tell when a high power transformer is performing in a strong electrostatic field?

  • Lu Chee Ket
    Lu Chee Ket 2 months ago

    Right hand screw rule is best to rely and consequently charges appear with relative motion induce equal charges to flow in opposite direction as equivalence to magnetic's in or out for determination of force direction.

  • Lu Chee Ket
    Lu Chee Ket 2 months ago

    The prevailed +dQ to moving +q repels, and the prevailed -dQ to moving +q attracts with opposite current seems to be logically right.

  • AvioPhile
    AvioPhile 2 months ago

    That is pretty, then in all reference frames we agree on the end result. How does it fit with electromagnetic waves?

    • 0MoTheG
      0MoTheG 1 month ago

      Where is the contradiction?

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